1. Field
The disclosed embodiments relate generally to the field of communications, and more specifically to methods and apparatus for optimizing search rate and finger assignment rate in a wireless communication system capable of receiving multiple signals.
2. Background
The field of communications has many applications including, e.g., paging, wireless local loops, Internet telephony, and satellite communication systems. An exemplary application is a cellular telephone system for mobile subscribers. (As used herein, the term “cellular” system encompasses both cellular and Personal Communications Services (PCS) system frequencies.) Modern communication systems designed to allow multiple users to access a common communications medium have been developed for such cellular systems. These modern communication systems may be based on Code Division Multiple Access (CDMA), Time Division-Multiple Access (TDMA), Frequency Division-Multiple access (FDMA), Space Division-Multiple Access (SDMA), Polarization Division-Multiple Access (PDMA), or other modulation techniques known in the art. These modulation techniques demodulate signals received from multiple users of a communication system, thereby enabling an increase in the capacity of the communication system. In connection therewith, various wireless systems have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile communication (GSM), and some other wireless systems.
In FDMA systems, the total frequency spectrum is divided into a number of smaller sub-bands and each user is given its own sub-band to access the communication medium. Alternatively, in TDMA systems, each user is given the entire frequency spectrum during periodically recurring time slots. A CDMA system provides potential advantages over other types of systems, including increased system capacity. In CDMA systems, each user is given the entire frequency spectrum for all of the time, but distinguishes its transmission through the use of a unique code.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard), and (4) some other standards.
In the above named CDMA communication systems and standards, the available spectrum is shared simultaneously among a number of users, and techniques such as soft handoff are employed to maintain sufficient quality to support delay-sensitive services, such as voice. Data services are also available. More recently, systems have been proposed that enhance the capacity for data services by using higher order modulation, very fast feedback of Carrier-to-Interference ratio (C/I) from a mobile station, very fast scheduling, and scheduling for services that have more relaxed delay requirements. An example of such a data-only communication system using these techniques, is the High Data Rate (HDR) system that conforms to the TIA/EIA/IS-856 standard (the IS-856 standard).
In contrast to the other above named standards, an IS-856 system uses the entire spectrum available in each cell to transmit data to a single user at one time. One factor used in determining which user is served is link quality. By using link quality as a factor for selecting which user is served, the system spends a greater percentage of time sending data at higher rates when the channel is good, and thereby avoids committing resources to support transmission at inefficient rates. The net effect is higher data capacity, higher peak data rates, and higher average throughput.
Systems can incorporate support for delay-sensitive data, such as voice channels or data channels supported in the IS-2000 standard, along with support for packet data services such as those described in the IS-856 standard. One such system is described in a proposal submitted by LG Electronics, LSI Logic, Lucent Technologies, Nortel Networks, QUALCOMM Incorporated, and Samsung to the 3rd Generation Partnership Project 2 (3GPP2). The proposal is detailed in documents entitled “Updated Joint Physical Layer Proposal for 1xEV-DV”, submitted to 3GPP2 as document number C50-20010611-009, Jun. 11, 2001; “Results of L3NQS Simulation Study”, submitted to 3GPP2 as document number C50-20010820-011, Aug. 20, 2001; and “System Simulation Results for the L3NQS Framework Proposal for cdma2000 1x-EVDV”, submitted to 3GPP2 as document number C50-20010820-012, Aug. 20, 2001. These are hereinafter referred to as the 1xEV-DV proposal.
A wireless communication system may comprise multiple mobile stations and multiple base stations. FIG. 1 exemplifies an embodiment of a wireless communication system with three mobile stations 10A, 10B and 10C and two base stations 12. In FIG. 1, the three mobile stations are shown as a mobile telephone unit installed in a car 10A, a portable computer remote 10B, and a fixed location unit 10C such as might be found in a wireless local loop or meter reading system. Mobile stations may be any type of communication unit such as, for example, hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link 14 from the base station 12 to the mobile stations 10 and a reverse link 16 from the mobile stations 10 to the base stations 12.
A radio receiver operates in a relatively hostile environment. A radio signal propagating from a transmitter to a corresponding receiver is subjected to scattering and reflections by objects and structures surrounding the transmitter and receiver. Structures, such a buildings, and surrounding terrain, such as walls and hillsides, contribute to the scattering and reflection of the transmitted signal. The scattering and reflection of the transmit signal results in multiple signal paths from the transmitter to the receiver. The objects that contribute to the multiple signal paths are centered about the receiver in a radius that is proportional to the receive signal wavelength. The contributors to the multiple signal paths change as the receiver moves.
A searcher element of the radio receiver scans received signals for multipath signals and assigns the multipath signals that meet a threshold to demodulation elements (also called fingers). The rate at which the searcher element scans for the received signals is the searcher rate and the rate at which the fingers are assigned multipath signals is the Finger Assignment Rate (FAR).
Increasing the searcher rate or the FAR may increase signal gain; however, doing so has a cost. It also increases utilization of a processor, which can take away from the processor being utilized for other tasks. It would be useful to increase the searcher rate and the FAR only when signal gain is needed. Signal gain is needed when there is fading in the multipath signals when the relative velocity between a transmitting device and a receiving device is slow. Thus, there is a need in the art for an apparatus and method that optimizes the search rate and finger assignment rate based on fading of multipath signals when the relative velocity is slow.